1. Field of the Invention
The present invention generally relates to magnetic recording media, magnetic storage apparatuses and methods of producing magnetic recording media, and more particularly to a magnetic recording medium having a magnetic recording layer with tilted anisotropy easy axes, a magnetic storage apparatus which uses such a magnetic recording medium, and a method of producing a magnetic recording medium by inducing the magnetic easy axes of the magnetic recording layer to a particular direction with a high degree of alignment while maintaining vertical columnar growth for the magnetic layer.
The magnetic recording medium having the magnetic recording layer with the tilted anisotropy easy axes is sometimes referred to as a tilted perpendicular magnetic recording medium.
2. Description of the Related Art
There are three possible modes for recording which depend on the orientation of the anisotropy easy axes of a magnetic recording layer relative to the recording direction. The most popular for rigid disk applications is the longitudinal mode in which the easy axes of the magnetic recording layer are parallel to the substrate surface (in-plane) in a random way or oriented where most of the axes point along the circumferential direction. Due to the high demagnetizing fields Hd at high densities, the perpendicular mode has also been proposed. For this perpendicular mode, the easy axes of the magnetic recording layer are normal to the disk surface (film normal), and the demagnetizing field Hd is an issue only at low densities or long bit lengths.
Early perpendicular magnetic recording media showed significant thermal decay at low densities due to poor alignment of the easy axes to the film normal. Improvements in the underlayer and the use of multilayers which derive their anisotropy from the interfaces resulted in perpendicular magnetic recording media with easy axes having a high degree of alignment. This is evidenced by narrow XRD rocking curves and squareness S=1 which greatly improved thermal stability as well as reduced DC noise, that is, the noise at very low densities.
Perpendicular recording is thought to replace longitudinal recording. By employing a perpendicular magnetic recording medium with a soft magnetic underlayer (SUL), higher write fields may be theoretically achieved from a single pole-type (SPT) head, which enables higher anisotropy media with good thermal stability. However, as the direction of the head field during writing is almost parallel to the easy axis, the switching field Ho is a large fraction of the anisotropy field Hk.
Mallary et al., “One Terabit per Square Inch Perpendicular Recording Conceptual Design”, IEEE Trans. Magn. Vol. 38, No. 4, pp. 1719-1724, July 2002 have proposed shields for the SPT head which gives a transverse component along the recording direction to the field aside from the vertical component. This reduces the overall field strength and complicates the already difficult head fabrication.
An interesting concept has been reported by Gao et al., “Magnetic Recording Configuration for Densities Beyond 1 Tb/in2 and Data Rates Beyond 1 Gb/s”, IEEE Trans. Magn. Vol. 38, No. 6, pp. 3675-3683, November 2002, wherein the anisotropy axes of the magnetic recording layer are uniformly tilted in the cross track (radial) direction at 45°. With the easy axis at 45° to the recording field, the switching field is close to one-half the grain anisotropy field. At 1 Tbit/in2, they estimated a 12 dB increase in signal-to-noise ratio (SNR) compared to conventional perpendicular recording. This arises mostly from the increase in the grain anisotropy due to the angled recording configuration. This case may be classified under the third mode which is the tilted or oblique mode.
Magnetic recording media with the easy axes tilted at a fixed angle relative to the head field has been used for many years for magnetic tape applications. Moreover, the magnetic recording medium proposed by Gao et al. has been proposed several years earlier by a U.S. Pat. No. 5,875,082 to Takayama et al. In Takayama et al., Ti was used as an underlayer and sputter-deposited at an oblique angle. CoCrTa was deposited at normal incidence but the resulting c-axes for both layers were tilted from the film normal and aligned along the radial direction.
Zheng et al., “Control of the tilted orientation of CoCrPt/Ti thin film media by collimated sputtering”, J. Appl. Phys., Vol. 91, No. 10, pp. 8007-8009, May 2002 proposed a tilted CoCrPt/Ti magnetic recording medium wherein the magnetic layer is deposited using a collimator similar to that proposed by a U.S. Pat. No. 5,804,046 to Sawada et al. with the angle of the slots tilted by 45°. In contrast to Takayama et al., the Ti underlayer was deposited largely at vertical incidence. The cross-sectional TEM image shows that the magnetic layer columns themselves are tilted similar to tape media. The problem with this tilted columnar structure is that the “footprint” of a magnetic grain is enlarged along the tilt direction. The read head senses larger magnetic grains compared to when the columns are vertical. The result is a broader transition width for a tilt along the recording direction or a larger cross-track correlation length if the columns are inclined perpendicular to the recording direction. Both result in higher medium noise.
Hee et al., “Tilted media by micromagnetic simulation: A possibility for the extension of longitudinal magnetic recording?”, J. Appl. Phys. Vol. 91, No. 10, May 2002 proposed a simulation study of using a ring-head with a tilted perpendicular magnetic recording medium wherein the tilt is along the recording direction. Hee et al. cite the work Zheng et al. as to how such a magnetic recording medium could be fabricated.
A typical perpendicular medium includes an underlayer and a magnetic layer. To realize large head fields, an SPT head may be used and an SUL added to the medium structure. The SUL becomes a “part” of the head structure with the magnetic recording medium in the “gap”. Preferably, this gap is as narrow as possible such that thin underlayers are necessary.
The underlayer may be made of a single layer such as Ti or several layers such as CoCr on Ti. For hcp magnetic layers, the underlayer may be hcp or fcc or a combination. fcc materials tend to easily form the (111) texture on which an hcp material grows with a (0002) texture. An amorphous layer may also be deposited on the SUL to prevent any structural information from the normally thick SUL (200 nm) to be propagated to the magnetic layer.
The magnetic layer may be made of CoCrPt alloys which have magnetocrystalline anisotropies coming from the bulk of the film. The magnetic layer may also be made of multilayers of Fe/Pt, Co/Pt, Co/Pd, or CoB/Pd where the anisotropy arises from the interfaces. For these multilayers, large anisotropy values and orienting the anisotropy axis along the film normal may be easily achieved. A large nucleation field Hn (>2000 Oe) may also be realized which makes the magnetic recording medium robust against head field erasure. However, due to the large exchange interaction between the film grains, noise is difficult to control. This makes CoCrPt alloys attractive although the anisotropy values cannot match those of the multilayers. But for the available head fields, CoCrPt alloys are sufficient.
Orienting the c-axes of hcp Co alloys along the film normal seems easy as the c-plane has the least surface energy, but S=1 is difficult to achieve. The deviation of the c-axes from the normal must be small. A measure of this is usually specified by Δθ of the (0002) peak in an XRD spectrum data. On a thin layer of amorphous Ti, a U.S. Pat. No. 6,283,893 to Futamoto et al. were able to grow CoCr with a (0001) texture. With process improvements, tight control of the c-axis orientation may now be achieved resulting in S=1, but a large nucleation field Hn is more difficult to achieve.
Recent reports on CoCrPtO and CoCrPt—SiO2 have shown that both S=1 and large nucleation field Hn may now be realized. For example, Velu et al., “Low Noise CoCrPtO Perpendicular Media With Improved Resolution”, presentation at TMRC, August 2002, have shown that a CoCrPt—O/Ru/SUL medium exhibits a high squareness S, a large nucleation field Hn (>1000 Oe), and good recording resolution. With a CoCrPt—SiO2 magnetic layer, A. Otsuki, “Development of Large-Capacity Perpendicular Magnetic Recording Media”, presentation at IDEMA, 2002 was able to realize very high linear densities, and 900 kfci patterns were observed with magnetic force microscopy. In the case of Velu et al., the Ru layer has a thickness of 25 nm to 30 nm. This Ru layer, however, needs to be drastically reduced to around <10 nm, in order to improve resolution, increase head fields, and reduce side erasure due to the spreading of the head field to the neighboring tracks.
From Gao et al., conventional perpendicular magnetic recording media may be further improved by tilting the anisotropy axes along a particular direction, such as the radial direction, but Gao et al. provided no suggestions on how this may be achieved. Present tape media including the media suggested by Zheng et al. have inclined columns which increase noise. Moreover, for the latter, the collimator used does not allow substrates that are disk-shaped. Takayama et al. proposed a way to make such media by depositing the Ti underlayer using masks similar to what a U.S. Pat. No. 4,776,938 to Abe et al. have earlier reported. However, the direction of the tilt along the film plane (or projection on the plane) is not fixed relative to the recording head. The Ti underlayer thickness was also too thick especially for the magnetic recording medium with a soft magnet underlayer.
The U.S. Pat. No. 6,183,893 to Futamoto et al. disclosed an advanced perpendicular medium structure with a first layer which may be amorphous, an hcp second underlayer, and two magnetic layers that may be separated by a nonmagnetic spacer layer such as Ru or Re. Such a perpendicular medium structure may be made with a soft magnetic underlayer (SUL) to be use with an SPT head. Overwrite for such head/medium system may be improved by tilting the c-axes from the film normal.
A Ru spacer layer is also employed in longitudinal magnetic recording media to induce antiferromagnetic coupling between two or more magnetic layers. This structure is called a synthetic ferrimagnetic medium (SFM). However, for this case, a very narrow range of Ru thickness is essential (0.6 to 0.9 nm). The result is a thermally stable magnetic recording medium on which high linear densities may be written. However, this SFM concept cannot be readily applied to perpendicular magnetic recording media since the antiferromagnetic coupling is significantly reduced in the case of the perpendicular magnetic recording media.